Updated requirement for South minimum version to be PEP 440 compliant.

Since version 8.0, setuptools is PEP 440 compliant. The current requirements definition for South use a syntax that was okay for pre-8.0 setuptools, but anyone using 8.0 or above will get a requirements conflict that South >0.8,<0.9 is required but 0.8.4 is installed on any subsequent runs. This small change fixes that.